Assuming an even base and straight sides, you can work out the water volume of any shape by multiplying its surface area by its depth. Do your measurements in cm, divide by 1000 and the result will be in litres.
A simple cube measuring 50x50x50cm holds 125 litres:
50 (length) x 50 (width) = 2500 (2500 sq cm surface area)
2500 (surface area) x 50 (depth) = 125000 (volume in millilitres)
125000 divided by 1000 = 125 (volume in litres)
Or in short: 50x50x50 divided by 1000 = 125 litres
If the depth was 60cm it would be: 50x50x60 divided by 1000 = 150 litres
Cubes and rectangles are easy, but odd shapes are more difficult to work out. The first thing to do is work out the surface area of the shape as outlined below:

Surface area of circles can be worked out by multiplying radius squared by 3.14 (approximate)
Radius is half the diameter. Radius squared is half the diameter multiplied by itself.
So for a circle with a diameter of 50cm (radius of 25cm) and depth of 60cm we do the following:
25 x 25 = 625 (radius squared)
625 x 3.14 = 1962.5 (surface area in sq cm)
1962.5 x 60 = 117750 (volume in millilitres)
117750 divided by 1000 = 117.750 litres or 118 litres rounded off
Or in short 25x25x3.14x60 divided by 1000 = 118 litres

For working out the surface area of very odd shapes we need to use trapezoids, which are four sided shapes with two parallel sides. To work out the surface area of a trapezoid we first add together the lengths of each parallel side then divide this by two:
Example: 50 + 70 = 120 divided by 2 = 60
Then we take the distance between the two parallel sides (20) and multiply this by our previous result:
20 x 60 = 1200
This is our surface area in sq cm (1200 sq cm) which can be multiplied by our depth (1200x60=72000) and divided by 1000 to give volume in litres (72 litres)
In short: (50+70 divided by 2=60)x20x60 divided by 1000 =72 litres

Now we can work out the surface of a trapezoid, all we need to do is divide our shape into trapezoid areas and work out the surface area for each section.
Trapezoid 1 - (70+70 divide by 2 = 70) x 20 = 1400
Trapezoid 2 - (70+40 divide by 2 = 55) x 45 = 2475
Trapezoid 1 - (95+60 divide by 2 = 77.5) x 40 = 3100
Now we need to add up all the surface areas and multiply by the depth and divide by 1000 to get a volume in litres:
Total area = 1400 + 2475 + 3100 = 6975 sq cm
So if our total surface area is 6975 sq cm, our volume at 60cm depth will be:
6975 x 60 divided by 1000 = 418 litres
To get more accurate results, use more trapezoids. If your depth varies you can work out the volume for each trapezoid individually and add the total together for an overall volume:

If the sides of the pond or other area you are measuring are sloping at a steady angle, simply work out the surface area of the bottom of the pond and the top, and use an average of the two to work out your volume.

The best way of working out volume with stepped sides is to divide the area up into simpler sections and work out the volume of each

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